I think your implementation is really, really cool. Similarly I spent a good half hour bumbling through Cosmopolitan. Love what you've built.

IMO, the fact that asm() is used for a bunch of things unrelated to inline assembly -- but to your point, instead for changing compiler behaviors is more of an argument for exposing additional __attributes__, annotations and so on, rather than adding explicit support for asm() to the standard. This violates the principal of least surprise, and IMO, serves to further confuse rather than bring some predictability to C.

I'd suggest also that C should embrace some movement in the standard rather than agreeing to leave things as is forever and stapling legs onto the octopus that is C++. There was a good writeup here a while ago from someone involved with the committee, that they've gotten to the point where introducing new warnings for obviously broken behavior is off the table because they want to be warnings-compatible from release to release.

What I'm saying is I would rather see exposed intrinsics, primitives and other meaningful source annotations than codifying the spooky action at a distance of an empty string asm() call in the C standard. And if we're going to modify the standard there's a lot of low-hanging fruit I'd love to see cleaned up first.

[edit] I'd also like to add that I agree with your thesis that if this can be built with the tools provided instead of modifying the standard, that the onus is on the proposer. Based on some of the other analyses here it seems like it can't really be done in a universal way, but I'm open minded.

Hmm, that’s actually a very interesting viewpoint on inline assembly, and it’s certainly something that I think may be useful to have. That being said, the ergonomics of actually using it are still kind of poor, even putting aside the strange register names and such, since it’s obviously not really designed to do this. And the other concern I would have is that compilers tend to be fairly conservative when seeing such constructs; I think even with operands specified as much as possible there are still substantial gaps in what you can express to the compiler and also how much the compiler actually cares that you marked a particular register set as being clobbered-it might just spill more or be conservative if it doesn’t want to deal with your constraints. I should probably look to see if compilers treat it better today.